The present invention relates generally to lithium-ion battery systems, and more particularly to a method and system for charge equalization in a lithium-ion battery system.
A typical lithium-ion battery for a spacecraft has twenty or thirty cells in series, which are, very sensitive to over-charging and over-discharging, thereby requiring complex protection circuits.
A common approach to charging/discharging the batteries uses a primary/redundant charger and a primary/redundant discharger for all cells that are in series. A voltage clamp circuit is used to protect the cells from overcharging, and a disconnect switch and bypass switch protect the cells from over-discharging.
This is a complex and costly approach to charging and equalizing lithium-ion batteries. There is a waste of valuable bus power in the charge mode when the voltage clamp circuit is protecting the cells from overcharging. Also, the clamp circuit typically requires additional heat sinking to dissipate heat. There is a strong dependency on electro-mechanical devices to protect the cells from over-discharging, and such devices have limited life cycles.
There is a need for a simple, low cost and effective method of charging and equalizing the cells of lithium-ion batteries used on board a spacecraft.
It is an object of the present invention to provide a method of charging and equalizing the cells of a lithium-ion battery. It is another object of the present invention reduce the cost and complexity of the charging and equalizing system.
It is a further object of the present invention to provide reliable and efficient control of cell charge equalization, eliminating the need for a separate charge equalization circuit for each cell. It is still a further object of the present invention to provide individual chargers that are used during periods of charging and discharging for balancing cells in a lithium-ion battery.
The present invention is a system and method for charging and equalizing a lithium-ion battery having a single discharger for all of the cells in a series and a separate charger for each cell in the series. In a charge mode, the cell charger can terminate charging or reduce the level of charging when a cell reaches a maximum charge threshold, thereby avoiding overcharging. In a discharge mode, the discharge controller will command the cell charger to resume charging a particular cell when that individual cell reaches a minimum discharge threshold.
These and other features of the present invention will be better understood with regard to the following description, appended claims, and accompanying drawings.